Streaming current detectors (SCD's) of the type described below are known for their inaccurate measuring of fluid samples at least for three reasons. One source of inaccuracy is that over a period of time, electrodes of a streaming current detector are covered with impurities and lose accurate readings. The second source is wear and tear of mechanical parts over a period of extended use. The third source of inaccuracy is inconsistent amplification factors. Thus, different streaming current detectors may yield inconsistent readings for the same sample fluid.
Streaming current detectors measure the charge on particles in a fluid sample stream. U.S. Pat. Nos. 3,368,144 --Gerdes, 3,368,145 --Gerdes, 3,369,984 --Gerdes et al, and 3,399,133 --Gerdes et al show streaming current detectors. U.S. Pat. No. 4,446,435 --Canzoneri and U.S. Pat. No. 4,825,169 --Carver show improvements on the streaming current detector. In the instruments shown and described in these patents, a reciprocating piston moves a sample fluid past measuring electrodes.
A direct comparison of readings from different detectors is useful. Application in parallel flow processes indeed requires such a direct comparison. Additionally, data from a streaming current detector could be directly correlated and compared to other electrokinetic data such as Zeta potential. To avoid inaccurate readings by streaming current detectors, the instruments must be calibrated on a regular basis.
In the past, some attempts were made to test the streaming current detectors by using tap water. The use of tap water as a standard solution has been directed to a determination of whether the streaming current detector fell within prescribed limits. Stated another way, the instrument was tested with tap water to make a "go/no go" determination of whether the instrument was operating properly. Tap water from the same source may be used as a standard solution for a short period of time. However, tap water from different sources has slightly different charge conditions. In fact, tap water from the same source does not have a constant charge condition over a course of time. Thus, it would be difficult to calibrate a streaming current detector with tap water.
In calibrating a streaming current detector of the above described type, it is an object of the invention to provide a simple yet accurate calibration method. This is important because it is beneficial to calibrate the streaming current detectors in the field.
Another object of the invention is to use a electrokinetically stable calibration solution with a long shelf life. Although the calibration solution may be mixed just prior to its use, it is more convenient to stockpile a solution and use it repetitively over a period of time.
A further object of the invention is that the calibration method is completed within a relatively short period of time. Time to prepare the calibration solution, clean the electrodes, and rinse with the calibration solution should be minimal to reduce the down time of the streaming current detector. For continuous measuring of fluid samples such as colloidal charge alteration control, the length of down time is critical.
Yet another object is to correlate the streaming current data to Zeta potential so that the measurement of the streaming current is confirmed.